Crosslinkable sulfonated copolymer and fuel cell including polymeric composition of the same

ABSTRACT

A sulfonated copolymer including a crosslinking functional group and a fuel cell including a polymer composition of the same are provided. The sulfonated copolymer including a crosslinking functional group can remarkably reduce methanol crossover and maintain superior dimensional stability and ionic conductivity by reducing swelling.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. patent application Ser. No.11/603,382 filed on Nov. 21, 2006, which claims priority to and thebenefit of Korean Patent Application No. 10-2006-0017241, filed on Feb.22, 2006, in the Korean Intellectual Property Office, the disclosure ofwhich is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a crosslinkable sulfonated copolymerand a fuel cell including a polymeric composition of the same.

2. Description of the Related Art

Fuel cells are electrochemical devices which directly transform chemicalenergy between hydrogen and oxygen which are contained in hydrocarbonmaterials such as methanol, ethanol, and natural gas into electricalenergy. The energy transformation process of fuel cells is veryefficient and environmentally-friendly, thereby drawing attention forthe past few years.

Fuel cells can be classified into Phosphoric Acid Fuel Cells (PAFC),Molten Carbonate Fuel Cells (MCFC), Solid Oxide Full Cells (SOFC),Polymer Electrolyte Membrane Fuel Cells (PEMFC), and Alkaline Full Cells(AFC) according to the type of electrolyte used. All fuel cells operateon the same principle, but the type of fuel used, operating speed, thecatalyst used, and the electrolyte used are different. In particular,PEMFCs are capable of being used in small-sized stationary powergeneration equipment or transportation systems due to their highreaction speed, low operating temperature, high output density, rapidstart-up, and output request variation.

The core part of a PEMFC is a Membrane and Electrode Assembly (MEA). AMEA comprises a polymer electrolyte membrane and two electrodes oneither side of the polymer electrolyte membrane, which independently actas a cathode and an anode.

The polymer electrolyte membrane acts as a separator, blocking directcontact between an oxidizing agent and a reducing agent, andelectrically insulates the two electrodes while conducting protons.Accordingly, a good polymer electrolyte membrane has high protonconductivity, good electrical insulation, low reactant permeability,excellent thermal, chemical and mechanical stability under normal fuelcell conditions, and a reasonable price.

In order to meet these requirements, various types of polymerelectrolyte membranes have been developed, and, in particular, a highlyfluorinated polysulfonic acid membranes such as a NAFION™ membrane, havebeen developed due to their excellent durability and performance.However, a NAFION™ membrane needs to be sufficiently moisturized andused at 80° C. or less to prevent moisture loss.

Moreover, in a Direct Methanol Fuel Cell (DMFC), an aqueous methanolsolution is supplied as a fuel to the anode and a portion ofnon-reactive aqueous methanol solution is permeated to the polymerelectrolyte membrane. The non-reactive aqueous methanol solution thatpermeates to the polymer electrolyte membrane causes a swellingphenomenon in the polymer electrolyte membrane, and the swelling isdiffused affecting a cathode catalyst layer. Such a phenomenon isreferred to as ‘methanol crossover’, that is, the direct oxidization ofmethanol at the cathode where an electrochemical reduction of hydrogenions and oxygen occurs, and thus a methanol crossover results in a dropin electric potential, thus causing a decline in the performance of theDMFC.

This issue is common in other fuel cells using a liquid fuel such as apolar organic fuel.

SUMMARY OF THE INVENTION

One embodiment of the present invention provides a sulfonated copolymerincluding a crosslinking functional group which has excellent ionicconductivity and can remarkably reduce methanol crossover.

Another embodiment of the present invention also provides a method ofpreparing the sulfonated copolymer including the crosslinking functionalgroup.

An embodiment of the present invention also provides a polymer resultingfrom the polymerization of the sulfonated copolymer.

One embodiment of the present invention also provides a polymerelectrolyte membrane which can remarkably reduce methanol crossoverwithout sacrificing ionic conductivity and has improved dimensionalstability.

An embodiment of the present invention also provides a membraneelectrode assembly including the polymer resulting from thepolymerization of the sulfonated copolymer.

A further embodiment of the present invention also provides a fuel cellincluding the polymer resulting from the polymerization of thesulfonated copolymer.

According to an embodiment of the present invention, a crosslinkablesulfonated copolymer is provided, including a polymerizable unsaturatedfunctional group at one end or both ends thereof, the crosslinkablesulfonated copolymer including: at least one aromatic ether repeatingunit; and an aromatic ether repeating unit including a sulfonic acidgroup or a sulfonate group.

According to an embodiment, the aromatic ether repeating unit includinga sulfonic acid group or a sulfonate group is at least one of repeatingunits represented by Formulas 1 to 3 below, and the degree ofpolymerization is in the range of 3 to 1,000:

wherein, each of R₁ through R₈ is independently hydrogen, a C₁₋₁₀ alkylgroup, or a C₆₋₃₀ aryl group, and at least one is —SO₃Y, wherein Y ishydrogen or an alkali metal; and X₁ is a single bond, —O—, —S—, —(C═O)—,—SO₂—, a substituted or unsubstituted C₆₋₃₀ arylene group, a substitutedor unsubstituted C₁₋₂₀ alkylene group, a substituted or unsubstitutedC₅₋₃₀ hetero arylene group, a substituted or unsubstituted C₁₋₂₀heteroalkylene group, a substituted or unsubstituted C₆₋₃₀ alkylarylenegroup, a substituted or unsubstituted C₆₋₃₀ heteroalkylarylene group, asubstituted or unsubstituted C₆₋₃₀ alkylheteroarylene group, asubstituted or unsubstituted C₈₋₃₀ alkylarylalkylene group, or asubstituted or unsubstituted C₁₃₋₃₀ arylalkylarylene group;

wherein, each of R₉ through R₂₀ is independently hydrogen, a substitutedor unsubstituted C₁₋₁₀ alkyl group, or a substituted or unsubstitutedC₆₋₃₀ aryl group, and at least one is —SO₃Y, wherein Y is hydrogen or analkali metal; and each of X₂ and X₃ is a single bond, —O—, —S—, —(C═O)—,—SO₂—, a substituted or unsubstituted C₆₋₃₀ arylene group, a substitutedor unsubstituted C₁₋₂₀ alkylene group, a substituted or unsubstitutedC₅₋₃₀ heteroarylene group, a substituted or unsubstituted C₁₋₂₀heteroalkylene group, a substituted or unsubstituted C₆₋₃₀ alkylarylenegroup, a substituted or unsubstituted C₆₋₃₀ heteroalkylarylene group, asubstituted or unsubstituted C₆₋₃₀ alkylheteroarylene group, asubstituted or unsubstituted C₈₋₃₀ alkylarylalkylene group, or asubstituted or unsubstituted C₁₃₋₃₀ arylalkylarylene group;

wherein, each of R₂₁ through R₂₄ is independently hydrogen, asubstituted or unsubstituted C₁₋₁₀ alkyl group, or a substituted orunsubstituted C₆₋₃₀ aryl group, and at least one is —SO₃Y, wherein Y ishydrogen or an alkali metal.

The polymerizable unsaturated functional group may be one of(meth)acrylate, styryl, cinnamate, furfuryl, vinyl, acetylene, epoxy,and cyanate based functional groups.

According to another embodiment of the present invention, a method ofpreparing a crosslinkable sulfonated copolymer is provided including:condensation polymerization of at least one aromatic diol-, dinitro-, ordihalide-based monomer with an aromatic diol-, dinitro-, ordihalide-based monomer containing a sulfonic acid group or a sulfonategroup; preparing sulfonated polyarylene ether having a hydroxyl group atends thereof by adding the hydroxyl group at both ends of the result ofthe condensation polymerization; covalently bonding the hydroxyl groupat the ends of the sulfonated polyarylene ether with a crosslinkingderivative.

According to another embodiment of the present invention, a curingreaction product of the crosslinkable sulfonated copolymer describedabove is provided.

According to another embodiment of the present invention, a polymerelectrolyte membrane including a curing reaction product of thecrosslinkable sulfonated copolymer of above, or a polymer matrix and acuring reaction product of the crosslinkable sulfonated copolymer ofabove is provided.

According to another embodiment of the present invention, a membraneelectrode assembly is provided including: a cathode including a catalystlayer and a diffusion layer; an anode including a catalyst layer and adiffusion layer; and a polymer electrolyte membrane interposed betweenthe cathode and the anode, wherein the polymer electrolyte membraneincludes a curing reaction product of the crosslinkable sulfonatedcopolymer of above.

According to another embodiment of the present invention, a fuel cell isprovided including: a cathode including a catalyst layer and a diffusionlayer; an anode including a catalyst layer and a diffusion layer; and apolymer electrolyte membrane interposed between the cathode and theanode, wherein the polymer electrolyte membrane includes a curingreaction product of the crosslinkable sulfonated copolymer of above.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other embodiments of the present invention will becomemore apparent by describing in detail exemplary embodiments thereof withreference to the attached drawing in which:

FIG. 1 is a schematic drawing of a membrane electrode assembly accordingto an embodiment of the invention.

DETAILED DESCRIPTION

A crosslinkable sulfonated copolymer according to an embodiment of thepresent invention includes a crosslinking functional group at both endsthereof. Thus, by using a polymeric compound prepared by curing thecrosslinkable sulfonated copolymer, a polymer electrolyte membranehaving high insolubility to water, low fuel crossover, such as methanolcrossover, preventing swelling due to fuel at a high temperature,excellent dimensional stability, high ionic conductivity, and lowdeformability can be formed.

In an embodiment, the crosslinkable sulfonated copolymer includes apolymerizable unsaturated functional group at one end or both endsthereof, the crosslinkable sulfonated copolymer includes: at least onearomatic ether repeating unit; and an aromatic ether repeating unitcontaining a sulfonic acid group or a sulfonate group.

In one embodiment, the mole ratio of the at least one aromatic etherrepeating unit and the aromatic ether repeating unit containing asulfonic acid group or a sulfonate group may be in the range of 99:1 to5:95, preferably in the range of 99:5 to 15:85, and more preferably inthe range of 90:10 to 55:45. When the mole ratio of the aromatic etherrepeating unit containing a sulfonic acid group or a sulfonate group isless than the above range, hydrogen ionic conductivity in a conductivemembrane may deteriorate and membrane resistance may increase. When themole ratio of the aromatic ether repeating unit containing a sulfonicacid group or a sulfonate group is greater than the above range,membrane efficiency may deteriorate due to high swelling andpermeability caused by water and fuel such as methanol.

In an embodiment, the aromatic ether repeating unit including a sulfonicacid group or a sulfonate group is at least one selected from the groupconsisting of repeating units represented by Formulas 1 to 3 below, andthe degree of polymerization is a number in the range of 3 to 1,000:

wherein, each of R₁ through R₈ is independently hydrogen, a C₁₋₁₀ alkylgroup, or a C₆₋₃₀ aryl group, and at least one of R₁ through R₈ is—SO₃Y, wherein Y is hydrogen or an alkali metal; and X₁ is a singlebond, —O—, —S—, —(C═O)—, —SO₂—, a substituted or unsubstituted C₆₋₃₀arylene group, a substituted or unsubstituted C₁₋₂₀ alkylene group, asubstituted or unsubstituted C₅₋₃₀ heteroarylene group, a substituted orunsubstituted C₁₋₂₀ heteroalkylene group, a substituted or unsubstitutedC₆₋₃₀ alkylarylene group, a substituted or unsubstituted C₆₋₃₀heteroalkylarylene group, a substituted or unsubstituted C₆₋₃₀alkylheteroarylene group, a substituted or unsubstituted C₈₋₃₀alkylarylalkylene group, or a substituted or unsubstituted C₁₃₋₃₀arylalkylarylene group;

wherein, each of R₉ through R₂₀ is independently hydrogen, a substitutedor unsubstituted C₁₋₁₀ alkyl group, or a substituted or unsubstitutedC₆₋₃₀ aryl group, and at least one of R₉ through R₂₀ is —SO₃Y, wherein Yis hydrogen or an alkali metal; and each of X₂ and X₃ is a single bond,—O—, —S—, —(C═O)—, —SO₂—, a substituted or unsubstituted C₆₋₃₀ arylenegroup, a substituted or unsubstituted C₁₋₂₀ alkylene group, asubstituted or unsubstituted C₅₋₃₀ heteroarylene group, a substituted orunsubstituted C₁₋₂₀ heteroalkylene group, a substituted or unsubstitutedC₆₋₃₀ alkylarylene group, a substituted or unsubstituted C₆₋₃₀heteroalkylarylene group, a substituted or unsubstituted C₆₋₃₀alkylheteroarylene group, a substituted or unsubstituted C₈₋₃₀alkylarylalkylene group, or a substituted or unsubstituted C₁₃₋₃₀arylalkylarylene group;

wherein, each of R₂₁ through R₂₄ is independently hydrogen, asubstituted or unsubstituted C₁₋₁₀ alkyl group, or a substituted orunsubstituted C₆₋₃₀ aryl group, and at least one R₂₁ through R₂₄ is—SO₃Y, wherein Y is hydrogen or an alkali metal.

In one embodiment, the sulfonic acid group or the sulfonate groupincluded in the repeating unit of any one of Formulas 1 to 3 can provideproperties such as increased conductivity and improved thermalproperties to the repeating unit. The sulfonate group, as a cation,includes an alkali metal, and examples are lithium, potassium sodium,etc.

In another embodiment, the mole ratio of the repeating unit of any oneof Formulas 1 to 3 while forming a copolymer may be in the range of 0.01to 0.95. When the mole ratio is greater than 0.95, the amount of thesulfonic acid group is too much, causing swelling due to fuel, highpermeability, and deteriorated membrane properties. When the mole ratiois less than 0.01, conductivity decreases. A copolymer including such arepeating unit may be any one of a block copolymer and a randomcopolymer.

In one embodiment, the repeating unit of Formula 1 including thesulfonic acid group or the sulfonate group may have a structure, forexample, represented by Formula 11 or 12:

In one embodiment, the repeating unit of Formula 2 including thesulfonic acid group or the sulfonate group may have a structure, forexample, represented by Formula 21:

In another embodiment, the repeating unit of Formula 3 may have astructure represented by Formula 31:

In an embodiment, the crosslinkable sulfonated copolymer includes apolymerizable unsaturated functional group at one end or both endsthereof which connect to another functional group in another chain toform a polymerization reaction product prepared by curing. Examples ofthe polymerizable unsaturated functional group are (meth)a crylate,styryl, cinnamate, furfuryl, vinyl, acetylene, epoxy, cyanate basedfunctional group, etc., but the polymerizable unsaturated functionalgroup is not limited thereto.

Besides including the repeating unit of any one of Formulas 1 to 3, thecrosslinkable sulfonated copolymer may include at least one aromaticether repeating unit which does not contain a sulfonic acid group or asulfonate group. Such an aromatic ether repeating unit includes at leastone aromatic ring in its backbone, and an ether group (—O—) at one endthereof. In an embodiment, the at least one aromatic ether repeatingunit may be, for example, at least one selected from the groupconsisting of repeating units represented by Formulas (a) to (w) below:

In one embodiment, the at least one aromatic ether repeating unit may berepresented by Formula 4 below:

wherein, each of R₂₅ through R₃₂ is independently hydrogen, asubstituted or unsubstituted C₁₋₁₀ alkyl group, or a substituted orunsubstituted C₆₋₃₀ aryl group; and X₄ is a single bond, —O—, —S—,—(C═O)—, —SO₂—, a substituted or unsubstituted C₁₋₁₀ alkylene group, asubstituted or unsubstituted C₁₋₁₀ heteroalkylene group, a substitutedor unsubstituted C₆₋₃₀ arylene group, or a substituted or unsubstitutedC₅₋₃₀ heteroarylene group.

In an embodiment, the at least one aromatic ether repeating unit ofFormula 4, for example, may be selected from the group consisting ofrepeating units represented by Formulas 41 to 43 below:

Hereinafter, a method of preparing the crosslinkable sulfonatedcopolymer will be described.

According to an embodiment, the crosslinkable sulfonated copolymer canbe prepared by: condensation polymerization of at least one aromaticdiol-, dinitro-, or dihalide-based monomer with an aromatic diol-,dinitro-, or dihalide-based monomer containing a sulfonic acid group ora sulfonate group; preparing sulfonated polyarylene ether having ahydroxyl group at ends thereof by adding the hydroxyl group at both endsof the polycondensate; covalently bonding the hydroxyl group at the endsof the sulfonated polyarylene ether with a crosslinking derivative.

The condensation polymerization may use any general polymer preparationmethod, and is not specifically limited. For example, first, the atleast one aromatic diol-, dinitro-, or dihalide-based monomer and thearomatic diol-, dinitro-, or dihalide-based monomoer containing asulfonic acid group or a sulfonate group are mixed in an appropriateratio. Next, in an embodiment, the mixture is dissolved in a solvent,polymerized with a metallic base salt catalyst such as potassiumcarbonate (K₂CO₃) to form a copolymer in a sulfonate group state. In oneembodiment, the polymerization may be performed at a temperature in therange of 140 to 220° C. for a time in the range of 1 to 36 hours.

The at least one aromatic diol-, dinitro-, or dihalide-based monomer isnot limited as long it is an aromatic ring compound having at least tworeactive functional groups. For example, the at least one aromaticdiol-, dinitro-, or dihalide-based monomer may be a compound of any oneof Formulas (a) to (w) below:

wherein each of R₁ and R₂ in Formulas (a) to (w) are independently ahydroxyl group, a nitro group, or a halogen group. R₁ and R₂ may be thesame or different.

The aromatic diol-, dinitro-, or dihalide-based monomer containing asulfonic acid group or a sulfonate group used in the condensationpolymerization includes at least two hydroxyl groups, nitro groups, orhalide groups in its molecule. The aromatic diol-, dinitro-, ordihalide-based monomer containing a sulfonic acid group or a sulfonategroup is not limited as long as at least one hydrogen atom issubstituted with a sulfonic acid group or a sulfonate group. Forexample, the aromatic diol-, dinitro-, or dihalide-based monomercontaining a sulfonic acid group or a sulfonate group may be at leastone selected from the group consisting of compounds having a structurerepresented by Formulas 1a to 3a.

wherein X₁, X₂, X₃, R₁ through R₂₄ are as described above; and Y₁ is ahydroxyl group, a nitro group, or a halide group.

In the method of preparing the crosslinkable sulfonated copolymer, thehydroxyl group in the aromatic diol-based monomer reacts with thehalogen group or the nitro group. In an embodiment, the mole ratio ofthe hydroxyl group and the nitro or halogen group may be in the range of4.0:6.0 to 6.0:4.0, preferably in the range of 4.8:5.2 to 5.2:4.8, andmore preferably, 5.0:5.0. If the mole ratio is outside the above range,a non-reactive monomer and a polymer having low molecular weight areformed, which adversely effect the properties of the crosslinkablesulfonated copolymer.

Also, in an embodiment the mole ratio between the aromatic diol-,dinitro-, or dihalide-based monomer and the aromatic diol-, dinitro-, ordihalide-based monomer containing a sulfonic acid group or a sulfonategroup may be in the range of 99:1 to 5:95, preferably in the range of95:5 to 15:85, and more preferably in the range of 90:10 to 55:45. Whenthe mole ratio of the aromatic diol-, dinitro-, or dihalide-basedmonomer containing a sulfonic acid group or a sulfonate group is lessthan 99:1, hydrogen ionic conductivity in a conductive membranedeteriorates and membrane resistance may remarkably increase. When themole ratio of the aromatic diol-, dinitro-, or dihalide-based monomercontaining a sulfonic acid group or a sulfonate group is greater than5:95, membrane efficiency may deteriorate due to increased swelling andpermeability due to water and fuel such as methanol.

However, when the compound containing the sulfonate is used to form acopolymer, a monovalent proton needs to be substituted with a hydrogenion to provide ionic conductivity to the copolymer. During substitution,a strong acid, such as dilute hydrochloric acid or dilute sulfuric acid,may be used.

After the condensation polymerization, the copolymer has the hydroxylgroup at its end. Here, a crosslinking functional group is covalentlybonded to obtain the crosslinkable sulfonated copolymer. The covalentbonding is performed in the presence of base such as NaH or amine at anambient temperature or a heating temperature of a reaction solution,using a (meta)acrylate-based compound, a styryl-based compound, acinnamate-based compound, a cyanate-based compound, an epoxy-basedcompound, or a vinyl-based compound. Finally, a crosslinkable sulfonatedcopolymer represented by Formula 10 below, having a substitutedpolymerizable unsaturated functional group for hydrogen in the hydroxylgroup can be obtained.

wherein R₁ is a polymerizable unsaturated functional group; Y ishydrogen or an alkali metal; —Ar—O— is an aromatic ether repeating unit;X is a single bond, —O—, —S—, —(C═O)—, —SO₂—, a substituted orunsubstituted C₆₋₃₀ arylene group, a substituted or unsubstituted C₁₋₂₀alkylene group, a substituted or unsubstituted C₅₋₃₀ heteroarylenegroup, a substituted or unsubstituted C₁₋₂₀ heteroalkylene group, asubstituted or unsubstituted C₆₋₃₀ alkylarylene group, a substituted orunsubstituted C₆₋₃₀ heteroalkylarylene group, a substituted orunsubstituted C₆₋₃₀ alkylheteroarylene group, a substituted orunsubstituted C₈₋₃₀ alkylarylalkylene group, or a substituted orunsubstituted C₁₃₋₃₀ arylalkylarylene group; a is a number in the rangeof 0.01 to 0.99; b and c are numbers in the range of 0 to 0.99, whereina+b+c=1 and b+c>0.01; and k1 is a number in the range of 3 to 1,000.

The crosslinkable sulfonated copolymer forms a polymer electrolytemembrane by curing, with a polymer matrix when required. The curing maybe performed using heat, light, or electron beams, but is not limited asany conventional curing method may be used. In one embodiment, when thecuring is performed using heat, the temperature may be in the range of40 to 120° C. When light is used, ultraviolet rays may be used.

In an embodiment, the curing may be performed by adding a polymerizationinitiator, and the polymerization initiator may be at least one selectedfrom the group consisting of benzoin ethyl ether, benzyldimethylketal,diethoxyacetophenone, and AIBN. In another embodiment, the amount of thepolymerization initiator may be in the range of 0.01 to 10 parts byweight based on 100 parts by weight of the crosslinkable sulfonatedcopolymer.

The polymer matrix is not limited as long as it is a polymer matrixconventionally used to form a polymer electrolyte membrane. Examples ofthe polymer matrix include sulfonated polyetheretherketone (SPEEK),sulfonated polyetherethersulfone (SPEES), sulfonated polyimide (SPI),polyimide, polybenzimidazole, polyethersulfone, polyetheretherketone,etc.

The polymer electrolyte membrane according to an embodiment of thepresent invention may have an interpenetration (IPN) structure of acured product of the crosslinkable sulfonated copolymer and anotherpolymer. The other polymer may be selected considering the propertiesthat need to improve in the cured product of the crosslinkablesulfonated copolymer. For example, when ionic conductivity needs to beimproved, a polymer having excellent ionic conductivity can be used.

A polymer electrolyte membrane according to another embodiment of thepresent invention, for example, may be impregnated with an ionicconductive material, such as phosphoric acid. Any impregnation methodsthat are well known to those of ordinary skill in the art may be used,and an example may be digesting the crosslinking sulfonated copolymermembrane in phosphoric acid.

According to another embodiment of the present invention, the polymerelectrolyte membrane may have a laminated structure of a polymermembrane formed of a cured product of the crosslinkable sulfonatedcopolymer and another polymer membrane in a plurality of layers. Theother polymer membrane may be formed of any polymer electrolyte membranematerials well known to those of ordinary skill in the art, based on theproperties that need to improve in the cured product of thecrosslinkable sulfonated copolymer.

The polymer electrolyte membrane including the cured product of thecrosslinkable sulfonated copolymer according to an embodiment of thepresent invention has properties such as improved mechanical propertiesof the membrane and insolubility to water, due to increased molecularweight caused by crosslinking between polymer chains. Hence, the polymerelectrolyte membrane membrane including the cured product of thecrosslinkable sulfonated copolymer can prevent swelling caused by fuel,even when the sulfonic acid group has high conductivity.

In one embodiment, the present invention provides a membrane electrodeassembly including: a cathode including a catalyst layer and a diffusionlayer; an anode including a catalyst layer and a diffusion layer; and apolymer electrolyte membrane interposed between the cathode and theanode, wherein the polymer electrolyte membrane includes a cured productof the crosslinkable sulfonated copolymer of the present invention.

In one embodiment, referring to FIG. 1, the membrane-electrode assembly10 includes a polymer electrolyte membrane 11, catalyst layers 12, 12′,and gas diffusion layers 13, 13′ disposed on the outside surfaces of thecatalyst layers 12, 12′.

The cathode and the anode may be formed of materials well known to thoseof ordinary skill in the art. Also, the polymer electrolyte membraneincludes a cured product of the crosslinkable sulfonated copolymer ofthe present invention. The polymer electrolyte membrane may be usedalone or in combination with other membranes having ionic conductivity.

In an embodiment, a fuel cell is provided including: a cathode includinga catalyst layer and a diffusion layer; an anode including a catalystlayer and a diffusion layer; and a polymer electrolyte membraneinterposed between the cathode and the anode, wherein the polymerelectrolyte membrane includes a cured product of the crosslinkablesulfonated copolymer of the present invention.

The cathode and the anode may be formed of materials well known to thoseof ordinary skill in the art. Also, the polymer electrolyte membraneincludes a cured product of the crosslinkable sulfonated copolymer ofthe present invention. The polymer electrolyte membrane may be usedalone or in combination with other membranes having ionic conductivity.

Conventional methods of preparing a fuel cell disclosed in variousdocuments can be used when preparing such a fuel cell, and thus detaileddescriptions thereof are omitted.

The present invention will be described in greater detail with referenceto the following examples. The following examples are for illustrativepurposes and are not intended to limit the scope of the invention.

EXAMPLE 1

wherein, the mole ratio m/n=1/1.

As shown in Reaction Formula 1, 5.406 g (23.68 mmol) of diol compound ofFormula 51, 2.583 g (11.84 mmol) of dihalide compound of Formula 52,5.000 g (11.84 mmol) of dihalide compound containing sulfonic acid saltof Formula 53, and 4.25 g of anhydrous K₂CO₃ were injected into a 250 mlthree-neck flask equipped with Dean-Stark trap. Then, 65 ml ofdimethylsulfoxide and 30 ml of toluene were added to the 250 mlthree-neck flask and mixed as a solvent.

The mixture was refluxed with nitrogen at 140° C. for 4 hours, and thenwater generated was removed. After removing water, toluene was removed.The reaction temperature was increased to 180° C., and at 180° C.,polymerization was performed for 16 hours. Subsequently, 0.54 g (2.368mmol) of diol compound of Formula 51 and 30 ml of toluene were added tothe resultant and then a reflux reaction was additionally performed for12 hours. Accordingly, a sulfonated copolymer of Formula 54 havinghydroxyl groups at both ends thereof was obtained.

The sulfonated copolymer of Formula 54 was cooled down to an ambienttemperature, and then precipitated into methanol. To remove inorganicmatter, the precipitated sulfonated copolymer was washed with hot waterthree times. Finally, the copolymer obtained was dried at 100° C. for 24hours.

EXAMPLE 2

5 g of sulfonated copolymer of Formula 54 obtained from Example 1 wasdissolved in 30 ml of dried 1-methyl-2-pyrrolidinone with nitrogen, andthen 1.42 g of NaH (60% dispersion in mineral oil) was added thereto.The result was stirred for 6 hours at an ambient temperature, and thencooled down using ice water. Subsequently, 5.4 g of methacryloylchloride was slowly added to the result. Then, the reaction wasperformed for 12 hours at the ambient temperature. The reactionprecipitate was separated through a glass filter, and then the filtratewas precipitated into a mixed solution of methanol and water. Next, theresultant was dried under a vacuum to obtain a methacrylic end-cappedcrosslinkable sulfonated polyetheretherketone copolymer of Formula 55above. The structure of the methacrylic end-capped crosslinkablesulfonated polyetheretherketone copolymer was analyzed using nuclearmagnetic resonance, and a characteristic peak of the methacryl group wasconfirmed as below.

1H NMR (DMSO-d6) (300 MHz) data: δ 5.8˜6.2 (2H, H2C═C—) and 2.0 (3H,C═C—CH3) (Mw=82,000, Mn=52,000)

EXAMPLES 3 TO 5

The crosslinkable sulfonated copolymer of Formula 54 obtained fromExample 2 and a copolymer of any one of Formulas 61 or 62 werecompletely dissolved in dimethylsulfoxide shown in Table 1, and theresulting mixture was cast onto a glass plate. Accordingly, the resultwas dried in an oven at 60° C. for 12 hours to prepare a polymerelectrolyte membrane. Next, the polymer electrolyte membrane was soakedin 1.5 M sulfuric acid solution for 24 hours for protonation.Subsequently, the product was immersed in distilled water for 24 hoursto obtain polymer membranes having the thickness shown in Table 1 below.

wherein, m/n=4/1.

wherein, a/c=5/95 and b/d=33.3/66.6.

COMPARATIVE EXAMPLE 1

A polymer membrane was prepared according to Example 3, except that onlyan acidic polymer of Formula 61 was used. The thickness of the obtainedpolymer membrane was 35 μm.

COMPARATIVE EXAMPLE 2

A polymer membrane was prepared according to Example 3, except that onlyan acidic polymer of Formula 62 was used. The thickness of the obtainedpolymer membrane was 109 μm.

The membrane thickness, proton conductivity using a 4-point probe cell,and methanol permeability of the above polymer membranes were measuredat the ambient temperature (25° C., RH=95%). The results are shown inTable 1 below.

TABLE 1 Polymer membrane elements Formula AIBN Acid polymer 55 AmountAmount Amount (Parts by Proton Methanol (Parts by (Parts by weight:compared to Thickness conductivity permeability Examples Kind weight)weight) Formula 55) (μm) (S/cm) (cm²/sec) Example 3 Formula 70 30 0.08464 4.16 × 10⁻³ 7.55 × 10⁻⁸ 61 Example 4 Formula 70 30 0.084 61 1.48 ×10⁻² 1.54 × 10⁻⁷ 62 Example 5 Formula 50 50 0.140 51 5.17 × 10⁻² 1.75 ×10⁻⁷ 62 Comparative Formula 100 — — 35 9.50 × 10⁻⁴ 7.46 × 10⁻⁸ Example 161 Comparative Formula 100 — — 109 5.26 × 10⁻³ 4.21 × 10⁻⁷ Example 2 62

As shown in Table 1, Examples 3 to 5, which are the polymer membranesincluding the cured products of the crosslinkable sulfonated copolymersaccording to the present invention, have improved proton conductivitywhile having the same methanol permeability value as before, compared toComparative Examples 1 and 2, which are the polymer membranes includingthe conventional acidity polymers.

By using the crosslinkable sulfonated copolymer and the cured productthereof, a polymer electrolyte membrane which can remarkably reducemethanol crossover and maintain superior dimensional stability and ionicconductivity can be obtained by reducing swelling caused by contact withliquid.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims and theirequivalents.

1. A method of preparing a crosslinkable sulfonated copolymercomprising: condensation polymerization of at least one aromatic diol-,dinitro-, or dihalide-based monomer with an aromatic diol-, dinitro-, ordihalide-based monomer containing a sulfonic acid group or a sulfonategroup; preparing sulfonated polyarylene ether having a hydroxyl group atends thereof by adding the hydroxyl group at both ends of the result ofthe condensation polymerization; covalently bonding the hydroxyl groupat the ends of the sulfonated polyarylene ether with a crosslinkingderivative.
 2. The method of claim 1, wherein the mole ratio between ahydroxyl group of the aromatic diol-based monomer and a hydroxyl group,a nitro group, or a halogen group of the aromatic diol-, dinitro-, ordihalide-based monomer containing a sulfonic acid group or a sulfonategroup is in the range of 4.0:6.0 to 6.0:4.0 in the condensationpolymerization.
 3. The method of claim 1, wherein the mole ratio betweenthe aromatic diol-, dinitro-, or dihalide-based monomer and the aromaticdiol-, dinitro-, or dihalide-based monomer containing a sulfonic acidgroup or a sulfonate group is in the range of 99:1 to 5:95.
 4. A methodof preparing a sulfonated copolymer cured product, the method comprisingforming a curing reaction product by curing a crosslinkable sulfonatedcopolymer obtained by using the method of claim 1 with a polymerizationinitiator.
 5. The method of claim 4 comprising forming a curing reactionproduct with a polymerization initiator.
 6. The method of claim 5,wherein the polymerization initiator is selected from the groupconsisting of benzoin ethyl ether, benzyldimethylketal,diethoxyacetophenone, AIBN, and combinations thereof.
 7. The method ofclaim 5, wherein the amount of the polymerization initiator is in therange of 0.01 to 10 parts by weight based on 100 parts by weight of thecrosslinkable sulfonated copolymer.